Many different types of plastic bag making machines are known in the art of producing plastic bags for industrial and individual consumers for many different applications (e.g. small sandwich bags and trash bags). A discussion of the history and operation of these machines can be found in U.S. Pat. No. 4,642,084 (hereby incorporated by reference) entitled "Plastic Bag Making Machine", assigned to the present assignee. The '084 patent discloses a bag machine which includes a rotary drum with seal bars attached thereto. Prior art machines maintain the position of the seal bars (at the drum periphery) using air cylinders. Improvements made to that design are described in other patents assigned to the present assignee, including U.S. Pat. Nos. 4,934,993, 5,518,559 and 5,587,032. Generally, the machines implementing these inventions have been well received.
A prior art bag making machine constructed generally in accordance with the descriptions of the above referenced patents is shown in FIG. 1. The major elements of such a prior art bag making machine 10 include a dancer and idler assembly 12, a sealing drum and blanket assembly 14, a chill roll 16, a controller 15, a punching station 17, a folding station 18, a pull roll system 20, a perforator/cutting station 22 and a phase variator assembly 24.
Film 11 is fed in the direction of the arrows from a source of plastic tubing 13 through a dancer roll 12b and an idler roll 12b into the sealing drum and blanket assembly 14. Source 13 may be any source for plastic material such as an extruder, a roll of plastic film, or a printer on which the plastic is imprinted. Dancer roll 12a exerts a known tension on the film by pulling it in a downward direction. The position of dancer roll 12a is used to determine the actual speed (by determining the difference from a nominal speed). The nominal tension is adjusted pneumatically. The adjustment may be difficult for the user because the adjustments are counter-intuitive: increasing pressure on the dancer cylinder decreases film tension.
The sealing drum and blanket assembly 14 includes a cylindrical drum 28, which is capable of being varied in diameter. That feature is illustrated by the dotted circle illustrating a smaller diameter. A number of sealing bars 30 are also shown and periodically form cross seals across the flattened film tube 11. Sealing bars 30 are of conventional design and are disclosed in detail with respect to construction and operation in the '084 patent. A blanket 32 is mounted on rollers 34, 35, 36 and 37 for surrounding a portion of drum 28 in such a way that the film 11 passes between blanket 32 and drum 28 while seals are being formed. Rollers 34 and 35 are mounted to an elongate frame 39 which is pivotable between the solid and dotted line positions shown in FIG. 1. Frame 39 includes a perpendicular plate 40 near its midsection, the latter being coupled to an air cylinder 42 having an extensible rod 43. It will be appreciated that extension of rod 43 causes rollers 34 and 35 to move to the dotted line position when the drum diameter decreases, thereby maintaining tension of blanket 32 against drum 28.
Roller 37 is driven from a gear motor 44 by belt 45 to drive blanket 32, and in turn blanket 32 will rotate drum 28 due to the tension between these components. Motor 44 includes an encoder 47 which generates a position signal each revolution of motor 44. Alternative encoder locations are on roller 37 or roller 38. A detector 23, such as an electric eye or magnetic sensor is positioned directly above drum 28 and generates a signal when a small metal or magnetic protrusion 26 on drum 28 passes, i.e. each revolution of drum 28. From the output of encoder 47 and detector 23 the circumference of drum 28 and the linear travel of film 11 bag are determined by controller 15. In an alternative embodiment encoder 47 may be mounted via a pulley to roller 37 or roller 36.
After passing chill roll 16, the film 11 next passes through an optional punching station 17 which punches preselected hole and handle configurations in the film. Thereafter, the film may be further processed as shown or in any other appropriate manner.
Variator system 24 is driven from a gear box 63 by belt 64. Gear box 63 is driven by drum 28 through belt 65. Variator system 24 also includes a pair of gears 66 and 67, used to vary the phase of the perforator/cutting station 22 and punching station 17, respectively, or any other type of downstream device.
Prior art rotary sealing drums in general perform better when the film is not under tension when it is sealed. However, as the film travels through other parts of the machine it is desirable to place it under tension to control and drive it. Thus, prior art machines overspeed the film as it approaches the sealing drum. The roll where the film contacts the blanket is referred to as the lay on roll, and it assists in overspeeding the film. The blanket is driven at the speed of the machine. As the blanket moves around the lay on roll the outside of the blanket (relative to the lay on roll) travels faster than the inside of the blanket (which is at the machine speed). As the film passes around the lay on roll, it is on the outside of the blanket. Since film 11 is in contact with the outside of blanket 32, it too travels faster than the machine speed. An analogous effect occurs when the film's contact with the blanket ends.
The prior art was thus limited in the ability to overspeed the film (i.e. it was determined largely by the blanket thickness. Additional control was obtained by an additional nip and the user needed to adjust the tension of the dancer rolls when adjusting the overspeed. Also, a mechanical adjustment was made when product size was changed. Another drawback was the blanket needed to be able to hold the film when oversped, yet it still needed to be slick enough to release the film after sealing.
Prior art machines have a variety of controllable parameters, including dancer tension and the overspeeding of the lay on roll and chill roll (the roll following the drum). These adjustments were individually made, and required the user to make mechanical adjustments. The adjustments were made by observing the film being processed, and adjusting the settings. Thus, it is difficult for inexperienced operators to make the proper adjustments.
FIG. 1 shows a perforator or cutter 22 (a knife) used to perforate the location demarking the end of one bag and the beginning of the next. The film path through the knife is horizontal, which causes difficulty in threading. Also, the knife is mechanically driven with a variator. The user adjusts for tension using a magnetic particle clutch or an ac vector drive. These adjustments are also made by observing the process.
According, a bag making machine that provides a dancer assembly exerting upward tension is desirable. Also, it will preferably have a sealing drum with infeed and outfeed nips, such as servo driven nips, that control the web speed to provide sealing with reduced or no tension. Such a machine will also have controls that allow an inexperienced user to operate the machine.